Deepwater jacket design method

ABSTRACT

A new jacket design method is disclosed, especially for deepwater water heavy jacket applications. The method utilizes a special type of air bags, called Ship Launching Air Bags (SLAB), to provide low cost temporary buoyancy used for jacket installation purpose only. A designer only needs to satisfy the jacket stiffness for the resistance of environmental and gravity loads without the consideration of jacket reserve buoyancy. The required jacket reserve buoyancy could be increased with the utilization of temporally attached SLABs during the jacket installation.

FIELD OF THE INVENTION

The disclosure relates generally to an improved jacket design method of offshore fixed platforms, more particularly for deepwater jacket applications.

BACKGROUND OF THE INVENTION

An offshore platform is generally composed of two sections: 1) a substructure such as a jacket for a fixed platform, and 2) a superstructure such as a deck to be installed on the top of a substructure.

A deepwater substructure, deeper than 60 meters (about 200 ft) in water depth, of a fixed platform is normally fabricated as a single unit with battered legs onshore in a horizontal orientation and then skidded onto a transport vessel or a launch vessel, towed to the installation site in a horizontal orientation, launched or lifted off from the vessel, and placed at the seabed before upending/ballasting of the jacket to a vertical position. Finally, foundation piles are driven to fix the jacket with the seabed by grouting or welding.

In a traditional deepwater jacket design, 8-12 ballast tanks are usually designed and placed at the four corner leg bottoms, 2-3 ballast tanks at each leg. Venting and flooding systems are installed to control these ballast tanks through a control center at the top of a deepwater jacket. The first function of these ballast tanks is for the upending operation. During the upending process, some of these ballast tanks are flooded in combination with a crane hook lifting to make the jacket upend from a horizontal floating position to a vertical floating position before sitting down at seabed. The second function of these ballast tanks is to provide required on-bottom weight for the jacket on-bottom stability once the jacket is placed at seabed with the crane hook released.

For some deepwater jacket designs, steel buoyancy tanks could be added to improve the jacket reserve buoyancy and/or jacket floatation satiability. Buoyancy tanks are usually placed at two jacket locations: 1) attached at the upper portion of jacket main legs near waterline in a vertical orientation to improve jacket vertical floatation stability through the increased water plane areas by these tanks; 2) attached at the top of a jacket as additional reserve buoyancy devices for the jacket horizontal floatation. There are two main disadvantages for the application of these steel buoyancy tanks. The first one is the inefficiency in the supply of net buoyancy. One ton of steel used for the construction of buoyancy tanks could usually produce three tons of buoyancy, hence only two tons of net buoyancy is produced. The second disadvantage of the steel buoyancy tanks is the high cost associated with these buoyancy tanks. The overall costs include cost of design, cost of construction, cost of connecting to the jacket members and the offshore operational costs which include the cutting off the tanks and the transporting back for these tanks with assigned vessels. In most cases, these tanks are for one time use only which makes the overall cost of each application very expensive. Therefore, buoyancy tanks are usually the last choice for the application in deepwater jacket designs.

Most large and heavy deepwater jackets are designed as launched jackets, the required jacket reserve buoyancy ((total buoyancy—total weight)/total buoyancy) for these jackets should be greater than 12 percent. In order to reach this required reserve buoyancy, the diameters of many jacket members are forced to be increased purely for the increase of jacket reserve buoyancy. The increase of jacket member diameters has little to do with jacket in-place structural stiffness considerations. In a typical design, the ratio of a jacket tubular member diameter (D) over the member wall thickness (t) is in the range of 30˜60, (D/t=30˜60). When a tubular member D/t=30, the member is neutrally buoyant. Any jacket tubular member with D/t>30, this member will become positively buoyant. The larger the D/t ratio of a member, the greater the net buoyancy for the member.

The negative impact of these large diameter tubular members is the increase of environmental loads such as wave loads and current loads, especially in the upper portion of the jacket such as at 40 meters (131 feet) below water surface known as the wave zone. Current force with current velocity acting on a tubular member is proportional to the square of the member diameter, and wave force with a wave particle acceleration acting on a tubular member has a linear relationship with the member diameter. Therefore, the increase of member diameter will cause the increase of environmental loads. As a result, the required jacket structural stiffness will need to be increased accordingly. Under current deepwater jacket design method, a designer searches for the balance between jacket in-place stiffness requirement and jacket installation reserve buoyancy requirement when determining tubular member diameters. Clearly the current method for deepwater jacket design is not an efficient one.

Therefore there is a need for an improved deepwater jacket design method that is more efficient in producing net buoyancy and cost effective.

SUMMARY OF THE INVENTION

An offshore jacket design method using non-steel buoyancy tanks is disclosed. A special type of air bags, called launching air bags (SLAB), is utilized as buoyancy tanks to replace the steel buoyancy tanks.

This jacket design method includes the utilization of SLABs as temporary buoyancy tanks in deepwater jacket installation, dividing the SLABs into a plurality of groups, each group has two or more SLABs, connecting SLABs in each group laterally to form a SLAB sheet, wrapping some SLAB sheets around jacket main legs near water surface in a near vertical orientation, and wrapping some SLAB sheets around horizontal members the of jacket near water surface. The jacket design using this method could eliminate conventional ballast tanks. Most jacket members between water surface and 40 meters below water surface could be designed as slender members, such as D/t=20˜25, to reduce environmental loads.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings described herein are for illustrating purposes only of selected embodiments and not all possible implementations, and are not intended to limit the scope of the present disclosure. For a further understanding of the nature and objects of this disclosure reference should be made to the following description, taken in conjunction with the accompanying drawings in which like parts are given like reference materials, and wherein:

FIG. 1A is a side view of a conventional Ship Launching Air Bag with different attachments;

FIG. 1B is a cross section view of a Ship Launching Air Bag with two attached side rings;

FIG. 2A is a side view of a launch barge loaded with a conventional deepwater jacket under a transportation condition;

FIG. 2B is a side view of a conventional deepwater water jacket in a horizontal floating condition at water surface after a launch operation, connected with a set of slings and a lifting hook from a crane vessel to prepare for an upending operation;

FIG. 2C is a side view of a conventional deepwater jacket sitting at seabed with all lifting slings removed and all ballasted flooded;

FIG. 3A is a side view of a launch barge loaded with one embodiment of a revised deepwater jacket configuration, equipped with multiple SLAB buoyancy tank groups as attached and temporary buoyancy means, under a transportation condition;

FIG. 3B is a side view of a launch barge loaded with one embodiment of a revised deepwater jacket configuration, equipped with multiple SLAB buoyancy tank groups as attached and temporary buoyancy means, during a launch operation;

FIG. 3C is a side view of one embodiment of a revised deepwater jacket configuration in a vertical self-upended floating condition at water surface after a launch operation, connected with a set of slings and a lifting hook from a crane vessel to prepare for a lowering and set-down operation;

FIG. 3D is a side view of one embodiment of a revised deepwater jacket sitting at seabed, with all lifting slings removed and all SLAB buoyancy tank groups removed/recovered after all airs inside SLAB tanks are released;

FIG. 4A is a front view of one embodiment of SLAB buoyancy tank “sheet” with five single SLABs connected with side rings to form a “sheet”, ready to wrap up a jacket leg member;

FIG. 4B is a side view of one embodiment of SLAB buoyancy tank “sheet” wrapping up a jacket horizontal member in a near vertical orientation;

FIG. 4C is a side view of one embodiment of SLAB buoyancy tank “sheet” wrapping up a jacket horizontal member in a horizontal orientation.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Ship building in sandy beaches started in 1980's in Southern China. Builders place wood blocks on a sloped sandy beach and start ship construction on the tops of these blocks with land cranes. When the construction is complete, a special type of air bags, Ship Launching Air Bags (SLAB), would be placed under the ship keel longitudinally between two rows of wood blocks. By injecting air to these SLABs, the whole ship would be lifted off these wood blocks. After the lifting operation, these wood blocks would be then removed off the ship keel. Once holding lines are cut, the ship would be launched toward the water on top of these rolling SLABs until the ship is afloat in water.

The ship launch method described above has been successfully deployed in China for quite some time already. Recently, the application of SLAB has expanded to ship salvage industry and offshore wet-tow transportation industry. In these applications, “ears” used for tying-up with other structures are added on the middle section surface of the SLAB. These “ears” usually use the same material such as nature rubber and polyester nets and experience a vulcanization process together with the middle section in order to be bonded together. Nowadays, the SLABs have become a mature, reliable and off shelf product in shipbuilding industry with excellent characteristics, such as light in weight, durable, scratch resistant, and tolerant of high internal pressure, etc.

In the present disclosure, SLABs are utilized as temporary buoyancy tanks to produce any required additional buoyancy for the jacket installation operation. Several SLABs could be grouped as one buoyancy unit and several units (8-16 units) may form a temporary buoyancy system with a control system located at the top of the jacket. With the utilization of SLABs in the jacket, a jacket designer only needs to consider the jacket structural stiffness for the resistance of environmental and gravity loads without worrying about satisfying the reserve buoyancy requirement based on steel tubular members alone. The designer may be able to design the jacket members in the wave zone as slender as possible (D/t=20˜25), while maintaining the required stiffness of the jacket for these members. In accordance with one embodiment, more than 80 percent of jacket members between water surface and 40 meters (about 130 ft) below water surface are slender members with D/t less than or equal to 30.

The SLAB buoyancy system disclosed herein not only provides sufficient reserve buoyancy for the jacket floatation and lowering operations after launched and self-upended, but also provides sufficient jacket on-bottom weight after the jacket is sit at the seabed with a crane hook released, by selectively release air from each buoyancy tank group of the buoyancy tank system. After the jacket installation is completed, any buoyancy tank group located above water surface may be released and recovered with the help of a lifting crane. Any submerged or semi submerged SLAB buoyancy tanks may be released and recovered with the help of a ROV (Remotely Operated Vehicle). For a launched jacket, all buoyancy tank groups may be placed near the upper portion of the jacket in order to suit a self-upending floatation configuration. In this configuration, four corner jacket main leg bottoms are free flooded during the launch. For a lifted jacket, four corner jacket main leg bottoms are closed to form ballast tanks. After reaching a stable floatation confirmation in a near horizontal orientation, an upending operation will be performed with a flooding operation, including the four corner jacket main leg bottom tanks, in combination with lifting actions by a hook load from a crane vessel.

The disclosed jacket design with SLAB buoyancy system has the following advantages:

1. More effective in producing net buoyancy—comparing with conventional steel buoyancy tanks in which each ton of steel-made buoyancy tanks could produce about 2-ton of net buoyancy, each ton of SLAB buoyancy tanks could produce more than 60-ton net buoyancy;

2. Easy installation and offshore removal without welding and offshore cutting-SLAB buoyancy tanks only require tying them up with jacket members which make the installation and offshore removal of SLAB buoyancy tanks easy. For underwater applications, ROV (Remote Operational Vehicle) could be used to cut off the, tic-up connections and recover SLAB buoyancy tanks without utilization of divers;

3. The detail locations of buoyancy distribution requirements of a deepwater jacket could be easily achieved by tying-up SLAB buoyancy tanks at the location where they are needed—much more flexible comparing with conventional steel buoyancy tanks or using the method with the increased diameters of some selected tubular members;

4. Each SLAB buoyancy tank group could be used to replace the functions of conventional ballast tanks at the bottom of jacket main legs—the selectively releasing of air from the submerged SLAB buoyancy tanks functions as the same as the reduction of buoyancy force for the jacket, which is equivalent to the function of putting ballast water in selected ballast tanks;

5. Reusable at low cost—SLAB is designed for multiple uses. Therefore, the total cost of SLAB buoyancy tanks could be a small fraction cost comparing with conventional steel buoyancy tanks for the same jacket installation applications.

The key issues in applying SLABs in deepwater jacket design are how to develop a tie-up method between SLAB buoyancy tanks and jacket members and how to select proper locations for the tied-up assembly, in which the tie-up connections should be strong enough to take potential environmental loads during jacket launch and transportation. In addition, these SLAB buoyancy tanks should also be easily released and recovered after an offshore jacket installation is complete.

There are two common functions for buoyancy tanks: 1) the increase of reserve buoyancy to the jacket during the jacket installation operation; 2) the increase of jacket floating stability through an enlarged water plane area of the jacket by these tanks during jacket floatation at water surface. Accordingly, two different tie-up methods are introduced in this disclosure: Type I method for Type I SLAB buoyancy tanks and Type II method for Type II SLAB buoyancy tanks. The main objective of the Type I tie-up method is to increase the jacket reserve buoyancy. The main objective of the Type II tie-up method is to increase the floating stability of a jacket. In general, these non-steel buoyancy tanks contribute more than 20 percent of the total reserve buoyancy of a jacket.

According to one embodiment, a number of SLABs are laterally connected to form a “sheet” through the side “rings”, two rows of rings at each side of one SLAB with equal distance apart. For Type I SLAB buoyancy tanks, this “sheet” then wraps up a jacket member in either a near vertical orientation such as jacket main legs or in a horizontal orientation such as jacket horizontal members to form one group of SLAB buoyancy tanks. These grouped buoyancy tanks may be placed in the proper locations to provide desired buoyancy distribution for the jacket floatation.

Type IISLAB buoyancy tanks are used to increase the jacket floating stability when the jacket is afloat, they are usually placed near the water surface area with jacket corner main legs. Type II SLAB buoyancy tanks are usually placed in a near vertical orientation, and may be tied-up with jacket main corner legs with the same “wrapped-up” method described above, near the upper portion of these jacket main legs.

A standard SLAB is made of a tubular middle section and two cone sections at the ends. FIG. 1A illustrates one embodiment of a standard SLAB 100. As shown in FIG. 1A, a standard SLAB 100 is divided into three sections: a front cone section 101, a middle section and a back cone section 101. The length of the middle section varies for each application. The middle section is made of nature rubber and multiple layers of polyester nets bonded together through a vulcanized process. During the SLAB 100 assembling process, the air bag is put into a sealed container with high temperature for a predetermined duration with a vulcanization process to make the rubber layers tightly bonded with the cone steel surfaces at both ends and the rubber bonded with layers of polyester nets at the middle section and the two end sections. At both ends of the SLAB, some attachments such as a pressure meter 104, steel ring(s) 106 and an air valve are installed. With this assembly, SLAB 100 becomes a flexible pressure vessel.

As shown in FIG. 1B, one pair of side steel rings 102 are attached at each side of the SLAB 100. Typically, several rows of the side ring pairs will be longitudinally arranged at the surface of SLAB 100 middle section. These side rings 102 are used for connecting SLABs

FIG. 2A through 2C illustrate a conventional deepwater jacket. Referring now to FIG. 2A, a launch barge 300, equipped with a pair of rocker arms 301 and a pair of launchways 302, loaded with a conventional deepwater jacket 200 with a total length longer than 60 meters in a transportation condition. The jacket 200 is primarily made of steel tubular members such as main leg members 203, horizontal members 201, diagonal members 202, and ballast tanks 204 inside jacket main leg 203 bottoms. Lifting padeyes 205 are attached at jacket main leg tops for an upending operation and a lowering operation.

Looking at FIG. 2B, after a launch operation or a lifting operation, the jacket 200 is afloat horizontally at water surface 310. A set of slings 313 are connected between a hook 311 from a crane vessel and padeyes 205 at jacket main leg 203 tops. At this stage, all ballast tanks 204 are empty.

Looking at FIG. 2C, the jacket 200 is upended and lowered to seabed 312. All ballast tanks 204 are flooded to help the jacket reach required on-bottom stability.

Referring to FIG. 3A, a launch barge 300, equipped with a pair of rocker arms 301 and a pair of launchways 302, loaded with a revised deepwater jacket 200 with a total length longer than 60 meters in a transportation condition. The jacket 200 is primarily made of steel tubular members such as main leg members 203, horizontal members 201, diagonal members 202, and several groups of SLAB buoyancy tanks 120 and 130. Some of SLAB buoyancy tank groups 120 are attached at jacket 200 main leg members 203 and some of SLAB buoyancy tank groups 130 are attached at jacket 200 horizontal members 201. Conventional ballast tanks could be eliminated for the installation of this revised jacket 200 configuration.

All SLAB buoyancy tanks 100 should be injected with air prior to sail. However, at the installation site prior to launch operation, all tank internal pressure may be checked and re-injected of air if necessary.

FIG. 3B illustrates the revised jacket 200 equipped with SLAB buoyancy tank groups 120 and 130 is in the middle of a launch operation.

Referring to FIG. 3C, after a launch operation or a lifting operation, the jacket 200 is afloat vertically at water surface 310 with four groups of SLAB buoyancy tanks 120 partially submerged to help the increase of jacket 200 water plane area for the floating stability. A set of slings 313 are connected between a hook 311 from a crane vessel and padeyes 205 at jacket main leg 203 tops.

A constant hookload will be maintained at the hook 311 throughout the lowering process. As the jacket 200 is lowered into the water, additional buoyancy from the jacket 200 buoyancy members will be added to the jacket 200 total buoyancy. Selectively releasing air from some SLAB buoyancy tanks 120 and 130 to balance the increase of buoyancy until the jacket 200 bottom sits at seabed 312.

Referring to FIG. 3D, after the jacket 200 sits at seabed 312, air will be released from all SLAB buoyancy tanks 120 and 130 to let the jacket 200 reach a designed on-bottom weight in order to satisfy the on-bottom stability requirement. The above mentioned actions could be achieved without the use of ballast tanks 204 at the jacket 200. All SLAB buoyancy tanks 120 and 130 will be removed and recovered from the jacket 200 for reuse for next application, with the help of a subsea ROV (Remote Operational Vehicle).

Referring now to FIG. 4A, one embodiment of SLAB buoyancy tank sheet is illustrated. In accordance with one embodiment, five SLABs are laterally connected using a connection wire 103 through side rings 102 to form a SLAB buoyancy tank sheet 110.

Referring to FIG. 4B, the SLAB buoyancy tank sheet 110 (including five laterally connected SLAB buoyancy tanks) wraps up a jacket main leg 203 to form a functional group of SLAB buoyancy tanks 120. This configuration could function as a group of Type I SLAB buoyancy tanks if it is submerged totally, or above water surface 310 totally, when the jacket 200 is afloat. This configuration could also function as a group of Type II SLAB buoyancy tanks if it is partially submerged at water line 310 when the jacket 200 is afloat.

At the lower end of the each SLAB buoyancy tank group 120, several padeyes 207 are welded at jacket main leg 203 surface. A wire 208 connects through all these padeyes 207 around the jacket main leg 203. A wire or a softline 107 is then connected between a steel ring 106 of a SLAB and wire 208. The wire 107 may be tightened by a shackle. When the jacket 200 installation is completed and SLAB buoyancy tanks are flat with some residual air inside, the wire 208 will be cut off by a ROV and all wires 107 should be free from the wire 208. The same ROV will also cut off the wires 103 for one column and the wire 208 at the top of the SLAB buoyancy tank group 120. After the cutting and with the residual buoyancy of buoyancy tanks 100, the whole group 120 will float to water surface 310 for recovery.

At the upper end of the each SLAB buoyancy tank group 120, a half pipe 206 is circularly around a jacket main leg 203 surface for the injection and the release of air for these SLAB buoyancy tanks 100. There is a steel pipe to connect this half pipe and a control panel located at the top of the jacket 200. All air injection and release are controlled by this control panel. At the surface of the half pipe 206, there are five stabbing connection receivers. A flexible hose 108 has one end connected to the valve at one SLAB top and another end with a stabbing connector connected to a stabbing connection receiver at the surface of the half pipe 206.

Referring to FIG. 4C, the SLAB buoyancy tank sheet 110 wraps up a jacket horizontal member 201 to form a functional group of buoyancy tanks 130. This configuration could be a group of Type I SLAB buoyancy tanks if it is totally submerged, or totally above water surface 310, when the jacket 200 is afloat. This configuration could also be a group of Type II SLAB buoyancy tanks if it is partially submerged when the jacket 200 is afloat. The basic configuration and function of this group of buoyancy tanks 130 is similar to that of the group of buoyancy tanks 120 described above.

The jacket design method disclosed herein satisfies all existing jacket design specifications in offshore industry. Compared with exiting deepwater jacket design method, this method will reduce environmental loads and the steel weight of the jacket when the jacket is subject to the same environmental conditions.

The present invention has been described in terms of specific embodiments incorporating details w facilitate the understanding of principles of construction and operation of the invention. Such reference herein to specific embodiments and details thereof is not intended to limit the scope of the claims appended hereto. It will be readily apparent to one skilled in the art that other various modifications may be made in the embodiment chosen for illustration without departing from the spirit and scope of the invention as defined by the claims. 

What is claimed is:
 1. A method for designing a jacket of an offshore fixed platform, the jacket having a plurality of jacket members including main leg members, diagonal members and horizontal members, the method comprising: preparing a plurality of non-steel buoyancy tank groups, wherein each buoyancy tank group comprises a plurality of air bags; and installing the prepared plurality of non-steel buoyancy tank groups on the jacket; wherein more than 80 percent of jacket members between water surface and 40 meters (about 130 ft) below water surface are slender members with D (member diameter)/t (member wall thickness) less than or equal to
 30. 2. The method according to claim 1, wherein each of the air bag is made of nature rubber and multiple layers of polyester nets bonded together through a vulcanized process.
 3. The method according to claim 1, wherein each of the air bag comprises a middle tubular section and two cone sections at each end.
 4. The method according to claim 3, wherein each of the air bag comprises a plurality of rows of side ring pairs longitudinally attached to the sides of the air bag middle section.
 5. The method according to claim 4, wherein the act of preparing a plurality of non-steel buoyancy tank groups comprises: dividing a plurality of air bags into a plurality of groups, each group comprising two or more air bags; and forming a sheet of air bags for each group by connecting adjacent air bags within the group laterally.
 6. The method according to claim 5, wherein the act of forming a sheet of air bags comprises connecting the plurality of rows of side rings of each air bag with the corresponding plurality of rows of side rings of an adjacent air bag by a plurality of wires.
 7. The method according to claim 5, wherein the act of installing the prepared plurality of non-steel buoyancy tank groups on the jacket comprises wrapping the prepared plurality of air bag sheets around the jacket main legs near water surface in a near vertical orientation.
 8. The method according to claim 5, wherein the act of installing the prepared plurality of non-steel buoyancy tank groups on the jacket comprises wrapping the prepared plurality of air bag sheets around some jacket horizontal members near water surface.
 9. The method according to claim 5, wherein the act of installing the prepared plurality of non-steel buoyancy tank groups on a jacket member comprises connecting a steel ring attached at the each end of an air bag to a wire circulating around the jacket member, wherein the wire circulates around the jacket member through a plurality of padeyes welded at the jacket member surface.
 10. The method according to claim 1, wherein the jacket does not employ any conventional ballast tanks inside jacket main leg bottoms.
 11. A method for installing a jacket at an offshore installation site, comprising: preparing a plurality of non-steel buoyancy tank groups, wherein each buoyancy tank group comprises a plurality of air bags; and installing the prepared plurality of non-steel buoyancy tank groups on the jacket; injecting air into each of the plurality of non-steel buoyancy tanks to achieve a first predetermined internal air pressure level; transporting the jacket to the installation site with a transportation apparatus; removing the jacket from the transportation apparatus, wherein the jacket becomes self afloat maintaining positive reserve buoyancy after removing from the transportation apparatus; releasing air from each of the plurality of non-steel buoyancy tanks; and removing the plurality of non-steel buoyancy tanks air bags from the jacket after the jacket installation is complete; wherein more than 20 percent of the jacket total reserve buoyancy is contributed by attached non-steel air bags.
 12. The method according to claim 11 further comprising reinjecting air into one or more air bags to achieve the first predetermined internal air pressure level after arrival at the installation site and before the jacket is off a transportation apparatus; .
 13. The method according to claim 11, wherein each of the air bag comprises a middle tubular section having a plurality of rows of side ring pairs longitudinally attached to the sides and two cone sections at each end.
 14. The method according to claim 13, wherein the act of preparing a plurality of non-steel buoyancy tank groups comprises: dividing a plurality of air bags into a plurality of groups, each group comprising two or more air bags; and forming a sheet of air bags for each group by connecting adjacent air bags within the group laterally.
 15. The method according to claim 14, wherein the act of forming a sheet of air bags comprises connecting the plurality of rows of side rings of each air bag with the corresponding plurality of rows of side rings of an adjacent air bag by a plurality of wires.
 16. The method according to claim 14, wherein the act of installing the prepared plurality of non-steel buoyancy tank groups on the jacket comprises wrapping the prepared plurality of air bag sheets around the jacket main legs near water surface in a near vertical orientation.
 17. The method according to claim 14, wherein the act of installing the prepared plurality of non-steel buoyancy tank groups on the jacket comprises wrapping the prepared plurality of air bag sheets around some jacket horizontal members near water surface.
 18. The method according to claim 14, wherein the act of installing the prepared plurality of non-steel buoyancy tank groups on the jacket comprises connecting a steel ring attached at the lower end cone section of an air bag to a wire circulating around a jacket member, wherein the wire circulates around the jacket member through a plurality of padeyes welded at the jacket member surface.
 19. The method according to claim 18, wherein the act of removing the plurality of non-steel buoyancy tank air bags from the jacket comprises cutting the connecting wire between each air bag and the wire circulating the jacket member by a ROV.
 20. The method according to claim 11, wherein the act of injecting and releasing air from air bags is conducted through a main control center above water surface.
 21. The method according to claim 11, wherein the installation site is a deepwater water location with a water depth greater than 60 meters (200 ft).
 22. The method according to claim 11, wherein the jacket does not employ any conventional ballast tanks inside jacket main leg bottoms.
 23. The method according to claim 11, wherein each of the air bag is made of nature rubber and multiple layers of polyester nets bonded together through a vulcanized process. 